1. Field of the Invention
The present invention relates to spring steels superior in fatigue properties, and springs obtained from the steels. The spring steels, if formed typically into high-strength springs, exhibit high fatigue properties and are useful as materials typically for valve springs in automobile engines, as well as clutch springs, brake springs, and suspension springs.
2. Description of the Related Art
With increasing demands on lighter weighs and higher outputs of automobiles, springs such as valve springs and suspension springs used typically in engines and suspensions are designed to be resistant to higher stress. These springs should therefore be superior in fatigue resistance and setting resistance so as to endure higher load stress. Among them, strong demands are made on valve springs to have higher fatigue strength, and such demands are not satisfied even by SWOSC-V (according to Japanese Industrial Standards (JIS) G3566) steel which has been believed to be superior in fatigue strength among known steels.
Spring steels need high fatigue strength, and thereby hard nonmetallic inclusions in the steels should be minimized. From this viewpoint, high-cleanliness steels that are minimized in the nonmetallic inclusions have been generally used for such applications. With increasing strength of material steels, risks of a break (disconnection) and fatigue fractures due to nonmetallic inclusions increase. Accordingly, nonmetallic inclusions mainly causing these defects should be more and more reduced in content and size.
A variety of techniques have been proposed for the reduction of hard nonmetallic inclusions in content and size in the steels. Typically, “182nd and 183rd Nishiyama Memorial Seminar”, The Iron and Steel Institute of Japan, pp. 131-134 (Reference 1) mentions that inclusions are finely divided upon rolling by maintaining the inclusions in a vitreous phase and that inclusions exist in a vitreous and stable composition in a system of CaO—Al2O3—SiO2. Japanese Unexamined Patent Application Publication (JP-A) No. Hei 05-320827, for example, mentions that it is effective to lower the melting points of inclusions so as to enhance deposition of the vitreous portion.
JP-A No. Sho 63-140068 mentions that a spring steel superior in fatigue properties is obtained by controlling the contents of Ca and Mg, and the total content of La and Ce within suitable ranges, controlling the chemical composition of the steel adequately, and adjusting the component ratios (component ratios of SiO2, MnO, Al2O3, MgO, and CaO) in the average composition of nonmetallic inclusions in the steel.
Japanese Examined Patent Application Publication (JP-B) No. Hei 06-74484 and JP-B No. Hei 06-74485 disclose such nonmetallic inclusion compositions as to make nonmetallic inclusions be liable to be drawn or destroyed upon cold working and be so soft as to cause substantially no fracture.
JP-A No. 2005-29888 proposes a technique of yielding a steel wire superior in fatigue strength, in which lithium (Li) is incorporated into the steel wire, thus inclusions have lower melting points and are enhanced to deform upon hot rolling.
These techniques indicate directions for improving characteristic properties such as fatigue properties. However, steels are not always maintained in a complete vitreous state and give crystals at temperatures for durations as employed in hot working, when the steels are controlled to have the compositions shown typically in Non-patent Document 1. Additionally, deposition of vitreous portions should be more and more enhanced so as to satisfy recent needs upon higher fatigue strength of steels.
Strict control of the average composition of inclusions is employed in most of the proposed techniques and exhibits some advantageous effects from the viewpoint of improving characteristic properties, such as fatigue properties, of spring steels. However, even according to these techniques, there may occur hard crystals such as high-SiO2 crystals and anorthite (CaO—Al2O3-2SiO2 oxide inclusions), and these cause fracture of the steel and adversely affect the fatigue properties thereof.